Take out the stem of he mushroom, some people scrape out the black scales I didn't, and either pan fry or bake. I popped mine in my toaster oven on 350 until cooked. Turn it over to get the juices out.

When cooked put it bottom side up, add tomato slice, crumbled bacon and cheese and put under broiler for a minute or leave in frying pan until cheese is melted.